The present invention relates generally to electroluminescent devices and more particularly to using an electroluminescent device as a radiation source for a scanner.
For a number of years, the best light source for scanning a document has been a fluorescent lamp. FIG. 1 shows such a tube fluorescent lamp. A large potential difference generated between the two electrodes at the ends of the tube breaks down a noble gas, such as argon, in the tube. Currents then conduct through the tube vaporizing and ionizing mercury droplets in the tube. When the mercury ions recombine after being excited, ultraviolet radiation is generated. The tube is coated by phosphors, which transform the incident ultraviolet radiation to visible light.
A fluorescent lamp is commonly used as the light source in an office document scanner because of its relatively low cost relative to prior art alternative light sources. However, the fluorescent lamp has some shortcomings when used for this purpose. Most notably, the fluorescent lamp is not a stable light source. It is an arc lamp, with light output highly dependent on the localized temperature dynamics of the arc, the noble gas and the vaporized mercury. Consequently, the light intensity from the lamp varies both spatially and temporally along the length of the lamp. Such variation degrades the accuracy of scanned images. Also, the fluorescent lamp should be warmed-up prior to use, as the heat generated from the arc has to vaporize and uniformly distribute the otherwise liquid drops of mercury. In addition, the fluorescent lamp is quite bulky and should be shielded to protect the scanner sensor from heat and stray light.
The problems are intensified in a color scanner as shown in FIG. 2A. In such a scanner, one typically needs three different broadband illuminators as the source to cover the visible spectrum. To scan the color of an area, each illuminator sequentially shines onto it. Reflections from each illuminator are measured to reconstruct the color of the area.
Normally, fluorescent lamp are broadband devices. Typically, the phosphors in each lamp are selected to irradiate in the red, green or blue of the visible spectrum, so that the three lamps fully cover the visible spectrum. In a prior art embodiment, the three lamps are put into an optical system so that they all illuminate a common scan line on an object, and the reflected_light is measured by a sensor. This system works, but may be inaccurate, wasteful and complicated because, in addition to all the above-identified difficulties of fluorescent lamps, the phosphors in each lamp age at different rates. This can lead to color error. Also, as shown in FIG. 2A, the light generated by each lamp is not directional. In scanning, one is looking at specific areas. The light that is not pointed towards those areas is wasted. In fact, such wasted light power usually tends to generate unwanted heat, which means that one needs to have thermal isolation.
FIG. 2B shows another prior art method using a single white light fluorescent lamp as the source of a typical scanner. In this example, the reflected beam is split into different paths to be measured by sensors that are sensitive to different colors. The difference in sensitivity to different colors may be achieved by placing different filters over the same type of sensors. This method again incurs the weaknesses of a fluorescent lamp.
Note that lasers or light-emitting-diodes (LEDs) are not very suitable as broadband illuminators. This is because both lasers and LEDs are inherently narrow-band devices. If the source is made up of a red, a green and a blue LED, color error may occur for an object area that is not primarily red or green or blue.
One excellent solution to the above problems has been proposed in a commonly assigned and co-pending U.S. application, titled, "An Edge Emitter As a Directional Line Source". That application describes different types of solid-state electroluminescent devices, all with edges. Each of them provides a broadband, directional, solid-state source that is stable, spatially and temporally uniform, rugged, efficient, compact and requires practically no warm-up period. The radiation generated emits from an edge of the device. If the devices are used in a scanner, it is very important for the devices to be stable spatially and temporally. In order to find out what one has scanned, typically, one compares the reflected radiation from the scanned surface with the incident radiation. A scanner would be much more expensive to accommodate for spatial and temporal variations in the incident radiation. Also, the devices in the co-pending application are very efficient because the radiation is emitted from edges over narrow lines. However, in some applications, one may not want to have the radiation confined to too narrow an area.
FIG. 3 illustrates one problem encountered by a very narrow source when it is used in a scanner. Typically, the medium scanned is planar, shown as dotted lines in FIG. 3. As light shines onto the planar medium, diffused light will be reflected and measured by a sensor. However, in many situations, one has to scan a medium with a curved surface, such as bound-printed subject matter close to the cusp of two adjacent pages. Due to the curvature of the medium, the light may impinge on a position quite different from the desired position; then the amount of the reflected diffused light reaching the sensor may be significantly reduced. One way to resolve the problem is to increase the power of the source and to widen the beam-width of the radiation through optics. However, this will increase the complexity and cost of the scanner.
It should be apparent from the foregoing that there is still a need for a broadband radiation source that is spatially and temporally uniform, with a beam-width more suitable to be used in a scanner, and that is relatively inexpensive to build.